This invention relates generally to machine tools, and more specifically to a system and apparatus for detecting worn or broken machine tools.
Automated machine tools, such as those used in turning gears, can be quite expensive devices which therefor advantageously employ methods and systems designed to minimize structural damage to them.
Several types of machine tools, including drilling and turning machine tools, operate at very high rotational speeds, sometimes exceeding 8000 revolutions per minute. Such high rotational speeds necessitate an apparatus which can detect a broken machine tool very quickly, so that incipient wear or breakage can be detected before significant damage can occur to the tooling machine on which the drill bit or rotating spindle is mounted. It is also desirable to quickly detect worn machine tool bits in order to assure a uniform high quality in the workpieces produced.
One method of detecting a worn or broken machine tool bit is based on the fact that for a machine tool drive system that drives the machine tool bit down on or into a workpiece, the force exerted by the machine tool on the workpiece will increase as the machine tool bit wears or breaks.
Typically, an automatic machine tool system comprises a mounting for the workpiece, a stationary member on which a carriage is movably mounted, and one or more machine tool bits mounted on the carriage in a way so that the bits may properly machine the workpiece. One method of detecting the force applied to the tool bit onto the workpiece is to incorporate piezoelectric transducer cells in the mounting of the tool bit to the carriage. Stresses placed on these load cells are translated into voltages across the cells, which are amplified and transmitted to an analog/digital converter. The analog/digital converter in turn supplies digital data, including information as to the intensity of the signal received and therefor the force applied to the load cell(s), to a calculator means. This force is then compared by the calculator means against a previous stored force limit to determine whether or not the machine tool bit has reached the end of its useful life. While the method described herein uses load cells placed in the tool mounting arrangement, it is obvious that these sensing transducers can be placed in any location that senses the forces between the tool and workpiece. One such embodiment would incorporate the transducers in the spindle bearings. Also other transducer types (strain gauges, etc.) could be used to sense the forces.
In the prior art, one of the more advanced systems for detection of machine tool wear or breakage is the PROMESS system, marketed in this country by Semtronics, P.O. Box 748, Brighton, Mich. 48116. This broken tool detection system is designed for operation with an automatic turning tool that uses several machining steps in an operation on a particular workpiece. For each of the machining steps, the receiving computer determines a normal force which occurs during the cut, and then calculates a tool break force limit by multiplying that average force by a predetermined constant. The break force limit thus calculated is valid for the remainder of only that particular machining step. In the PROMESS system, each machining step, whether or not such machining step may be a repetition of an earlier machining step, requires a separate tool break force limit calculation. One drawback of this system is that there is a short period of time from the beginning of a cut in any one machining step and the establishment of a tool break force limit for that step in which the machine is not protected against a tool breakage except for a manually set overload limit. If a tool breaks within this beginning period, the computer may allow damage to the machine tool system.
An additional problem unsolved by prior art involves the significant range of forces during rotation of the workpiece. This is generally caused by a variation in diameter with rotational position due to imperfection in the workpiece.